Heretofore, a ceramic product of honeycomb shape, or a ceramic honeycomb structure, has been made by preparing a ceramic green body through mixing of ceramic materials with a molding aids and a pore-forming agent, molding the body into a honeycomb-shaped ceramic green body through extrusion of the mixed body, and finally firing the honeycomb-shaped ceramic green body in a firing furnace at a predetermined temperature.
The molding aids to be mixed with the ceramic materials for the above firing of the honeycomb structure are for instance an organic binder, such as methyl cellulose, carboxymethylcellulose, polyvinyl alcohol, starch paste, flour, glycerin, and the like or a surface active agent, or wax, etc. The above pore-forming agent to be used with the ceramic materials is for instance graphite, saw dust, starch etc. Such molding aids and pore-forming agent have the following special properties; namely, the molding aids or pore-forming agent is hard to heat from the outside of the honeycomb structure, but once ignited, burns rapidly and generates heat (the igniting temperature varies depending on the kind of auxiliary). Thus, the rapid burning of the molding aids or pore-forming agent results in uneven temperature distribution in the structure and tends to cause cracks within or at end portions of the final ceramic honeycomb structure.
To prevent the occurrence of cracks, it has been practiced to use a slow rate when raising the temperature of the body being fired, so as to suppress rapid heat generation within it. However, the oxygen concentration in the firing atmosphere at a temperature in ignition ranges of the molding aids or pore-forming agent varies depending on fluctuation of the weight of ceramic green body loaded in the furnace (to be referred to as "the loaded weight", hereinafter). Hence, the manner in which the molding aids or pore-forming agent within the ceramic honeycomb body burns varies, so that the temperature difference between the inside and outer portions of the ceramic honeycomb body becomes large or small. In consequence, sometimes cracks generate but sometimes not, resulting in a problem that the quality of the product is not uniform and stable production is hard to realize.
In the firing process, the pore-forming agent comes in contact with oxygen of the firing atmosphere and burns out in a certain time period, so as to produce fine holes of a desired diameter. It has been a practice to terminate the firing process with a certain time relationship with the timing of the burn out of the pore-forming agent. However, when the above-mentioned cracks are generated, even if the loaded weight is the same at the temperature range for burning the pore-forming agent, the oxygen concentration in the firing atmosphere varies and the kind of pore-forming agent varies, so that the burning characteristics of the pore-forming agent changes. Hence, it has been experienced in actual production that a pore-forming agent within the ceramic honeycomb body sometimes burns out but sometimes not, and there is a problem that stable production of fired goods with uniform distribution of fine holes having a desired diameter throughout the inside and outer portions thereof is difficult to attain.
Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art by providing an improved method of firing ceramic honeycomb structures which ensures stable production of a high-quality product.